Wireless base stations are well known in the art and typically include, among other things, baseband equipment, radios and antennas. The antennas are often mounted at the top of a tower or other elevated structure such as a pole, rooftop, water tower or the like. Typically, multiple antennas are mounted on the tower, and a separate baseband unit and radio are connected to each antenna. Each antenna provides cellular service to a defined coverage area or “sector.”
FIG. 1 is a highly simplified, schematic diagram that illustrates a conventional cellular base station 10. As shown in FIG. 1, the cellular base station 10 includes an antenna tower 30 and an equipment enclosure 20 that is located at the base of the antenna tower 30. A plurality of baseband units 22 and radios 24 are located within the equipment enclosure 20. Each baseband unit 22 is connected to a respective one of the radios 24 and is also in communication with a backhaul communications system 44. Three sectorized antennas 32 (labelled antennas 32-1, 32-2, 32-3) are located at the top of the antenna tower 30. Three coaxial cables 34 (which are bundled together in FIG. 1 to appear as a single cable) connect the radios 24 to the respective antennas 32. Each end of each coaxial cable 34 may be connected to a duplexer (not shown) so that both the transmit and receive signals for each radio 24 may be carried on a single coaxial cable 34. It will be appreciated that in many cases the radios 24 are located at the top of the tower 30 instead of in the equipment enclosure 20 in order to reduce signal transmission losses.
Cellular base stations typically use directional antennas 32 such as phased array antennas to provide increased antenna gain throughout a defined coverage area. A typical phased array antenna 32 may be implemented as a linear array of radiating elements mounted on a panel, with perhaps ten radiating elements per linear array. Typically, each radiating element is used to (1) transmit radio frequency (“RF”) signals that are received from a transmit port of an associated radio 24 and (2) receive RF signals from mobile users and feed such received signals to the receive port of the associated radio 24. Duplexers are typically used to connect the radio 24 to each respective radiating element of the antenna 32. A “duplexer” refers to a well-known type of three-port filter assembly that is used to connect both the transmit and receive ports of a radio 24 to an antenna 32 or to a radiating element of multi-element antenna 32. Duplexers are used to isolate the RF transmission paths to the transmit and receive ports of the radio 24 from each other while allowing both RF transmission paths access to the radiating element of the antenna 32, and may accomplish this even though the transmit and receive frequency bands may be closely spaced together.
In order to transmit RF signals to, and receive RF signals from, a defined coverage area, each directional antenna 32 is typically mounted to face in a specific direction (referred to as “azimuth”) relative to a reference such as true north, to be inclined at a specific downward angle with respect to the horizontal in the plane of the azimuth (referred to as “tilt” or “elevation”), and to be vertically aligned with respect to the horizontal (referred to as “roll”). Unintended changes in azimuth, tilt, and roll can detrimentally affect the coverage of a directional antenna 32. Unfortunately, high winds, vibrations, corrosion or various other factors may cause the azimuth, tilt and/or roll of an antenna 32 to change over time. Accordingly, wireless service providers may monitor antennas 32 at cellular base stations 10 to identify when antennas 32 are no longer pointed in a desired direction.
In some cases, the antennas 32 may be mounted on motorized gimbals, and hence an operator can adjust the pointing direction of the antenna 32 from a remote location by sending control signals to the motorized gimbal. Additionally, some antennas 32 are designed so that the “electronic tilt” of the antenna 32 may be adjusted from a remote location. With antennas 32 that include such an electronic tilt capability, the physical orientation of the antenna 32 is fixed, but the effective angle of the antenna beam can be adjusted electronically by, for example, controlling phase shifters that adjust the phase of the signal fed to each radiating element of the antenna 32. The phase shifters and other related circuitry are typically built into the antenna 32 and can be controlled from a remote location. Typically, the phase shifters are controlled using Antenna Interface Standards Group (“AISG”) control signals, which are an industry standardized set of control signals used for controlling antennas used in cellular communications systems. Typically, the electronic adjustment of the antenna beam is used to change the downward angle or “tilt” of the antenna beam. Antennas 32 having beam patterns whose tilt angle can be adjusted electronically from a remote location are typically referred to as Remote Electronic Tilt (“RET”) antennas.
With RET antennas, a first phase shifter is used for the transmit frequency band and a second phase shifter is used for the receive frequency band. As separate transmit and receive phase shifters are used, the duplexers that are used to allow each radiating element to both transmit and receive signals must necessarily be located along the transmission path between the phase shifters and the radiating elements. With RET antennas, the phase shifters are typically mounted on the back side of the antenna panel, in very close proximity to the radiating elements. Consequently, the duplexers are also typically mounted on the back side of the antenna panel. As the number of radiating elements has increased (to provide better antenna gain patterns), this has made it more difficult to find room to mount the duplexers and other RF equipment and associated electronics on each antenna panel.
FIG. 2 is a perspective view of a conventional duplexer 50. FIG. 3 is a perspective view of the conventional duplexer 50 of FIG. 2 with the cover plate removed therefrom. FIG. 4 is a top perspective view of a portion of the housing of duplexer 50.
Referring to FIGS. 2-4, the conventional duplexer 50 is implemented as a three port resonant cavity filter. The duplexer 50 includes a housing 60 that has a floor 62 and a plurality of sidewalls 64. An interior ledge 66 is formed around the periphery of the housing 60. A plurality of internal walls 68 extend upwardly from the floor 62 to divide the interior of the housing 60 into a plurality of cavities 70. Coupling windows 72 are formed within the walls 68, and these windows 72 as well as openings between the walls 68 allow communication between the cavities 70. A large number of internally-threaded columns 74 are formed in the walls 68. A plurality of resonating elements 76 are mounted within the cavities 70. The resonating elements 76 may comprise, for example, dielectric resonators or coaxial metal resonators, and may be mounted by screws 80 onto selected ones of the internally threaded cavities 74 that are formed in the walls 68. A cover plate 78 acts as a top cover for the duplexer 50. A large number of additional screws 80 are used to tightly hold the cover plate 78 into place so that the cover plate 78 continuously contacts the interior ledge 66 and the top surfaces of the walls 68 to provide good performance with respect to Passive Intermodulation (“PIM”) distortion.
An input port 82 may be attached to an output port of a transmit path phase shifter (not shown) via a first cabling connection 83. An output port 84 may be attached to an input port of a receive path phase shifter via a second cabling connection 85. A common port 86 may connect the duplexer 50 to a radiating element of the antenna (not shown) via a third cabling connection (not shown). A plurality of tuning screws 90 are also provided. The tuning screws 90 may be adjusted to tune aspects of the frequency response of the duplexer 50 such as, for example, the center frequency of the notch in the filter response. It should be noted that the device of FIGS. 2-4 comprises two duplexers that share a common housing, which is why the device includes more than three ports (the device includes a total of six ports, although all of the ports are not visible in the views of FIGS. 2-4).
The conventional duplexer 50 of FIGS. 2-4 may provide acceptable performance. However, the duplexer 50 may be relatively large, and hence it may be difficult to make room to mount a large number (e.g., ten) of these duplexers 50 on a single flat panel phased array antenna. The duplexer 50 may also be relatively heavy, which increases the loading on the antenna. The duplexer 50 also has a large number of parts making fabrication and assembly more expensive.